The Japanese Foreign Ministry said it has lodged an official protest with the U.S. government.

The U.S. Marine Corps acknowledged the detention in a statement, without naming Hadnott. The incident was under investigation, the Marine Corps said, adding that it "takes this allegation very seriously."

A small group on Monday protested outside the building where authorities were holding Hadnott.

The assault was reported Sunday night in the Chatan entertainment district on Okinawa, a major hub for U.S. forces in the Pacific, police said.

Hadnott met the girl in Okinawa, offered her a ride and is alleged to have raped her in a car, according to local media reports.

Hadnott was detained shortly afterward, police said.

"This type of crime violates the human rights of women. And taking the fact that the victim was a junior high school student, this kind of crime cannot be tolerated," Okinawa Gov. Hirokazu Nakaima said in a statement.

Previous criminal cases against American troops have triggered widespread protests by Okinawans since 1995, when three American servicemen were found guilty of raping an Okinawan schoolgirl.

"The Okinawan Prefecture has repeatedly demanded a strict measure against these kinds of violent crimes by the U.S. military personnel every time it happens," Nakaima said. "Unfortunately, the crime has happened again."

Under U.S.-Japanese military agreements, the U.S. government has custody over service members suspected of crimes while on duty. In addition, the U.S. military is not required to turn over those accused of crimes until an indictment is issued.E-mail to a friend